The loads connected to the general city AC power, except the pure resistance loads such as incandescent lamps and electric stoves, most other loads include resistance and inductance. Hence the phase angle of the current of the electronic circuit lags the voltage. In general, the total current provided by the generator of the electric company through the electric transmission and distribution lines can be divided in effective current and ineffective current. As the rated voltage of each load is different, the electric meter at the user end indicates only the effective power (KW). But the ineffective power (KVAR) will cause line potential drop and line loss. This is a loss to the electric power company and user end. Hence how to adjust the power factor is an issue many electric user end system vendors are trying to resolve.
The technique of adjusting power factor can be divided into active type and passive type. Applying power factor adjustment can reduce electric loss, improve electric supply quality, increase life span of loads and save electricity consumption. The conventional approach for improvement is using capacitors. For instance, referring to FIG. 1, the present power supply vendors mostly provide a power factor adjustment device to stabilize output voltage. The drawing shows the operation voltage waveform of an inverter. After adjusting, the operation voltage becomes a continuous wave. But such an application mode creates troubles in some industries.
Take the inverter of a piezoelectric transformer as an example. As the operation output power of the inverter of the piezoelectric transformer is determined by the input voltage, with the present power supply mostly outputting a smaller voltage such as 24 or 48 volts, the output power of the inverter of the piezoelectric transformer also decreases. Moreover, the inverter of the piezoelectric transformer generates piezoelectric effect through vibration and mechanical stress of piezoelectric plates, when the power supply is activated, the piezoelectric plates have to maintain operation at 48 volts. As a result, the piezoelectric plates have to operate under a high voltage oscillation. This tends to shorten the life span of the inverter of the piezoelectric transformer.